How to adjust the position of a text on a plot?

I want to adjust the postion of the text S0=(1,1,0,0) and to put each in fron of a,b and c in the subplots. I also want to redue the space between the subplots denoted 1 and 2 enclosed by red rectangles. How can I do it? My program and figure can be seen:

clc;
clear all;
close all;
%% ------------------------------Program-------------------------------------
N0=100;
r_med=[0.445 0.889 1.445 2];
sigma_g=7;
N_ang=9001;
theta=0:0.01:180;
col=['k' 'b' 'r' 'g'];
for i=1:length(r_med)
    [P11(i,:),P12(i,:),P33(i,:),P34(i,:),~,~,~] = ZK_W_Cloud_PhaseFunc(N0,r_med(i),sigma_g,N_ang)
    f1(i,:)=((P11(i,:)-P12(i,:))./2);
    f2(i,:)=((P11(i,:)+P12(i,:))./2);
    f3(i,:)=P33(i,:)/2;
    %numer(i,:)=0.125*((f2(i,:).*(cosd(theta)).^2)+f1(i,:)-(2.*(f3(i,:).*cosd(theta))));
    denomin(i,:)=0.125*((3.*(f2(i,:).*(cosd(theta)).^2))+(3.*f1(i,:))+(2.*(f3(i,:).*cosd(theta))));
    %ldpr(i,:)=smooth((numer(i,:)./denomin(i,:)));%Linear depolarization ratio
    
    figure(1)
    hold on
    subplot(1,3,1);
    plot(theta,denomin(i,:),'color',col(i),'LineWidth',2);
    xlabel('\fontname{Arial}Scattering angle \theta(\circ)','FontSize',14,'FontWeight','normal');
    ylabel('\fontname{Arial}Phase function P_{L\mid\mid}(\theta)(sr^-^1)','FontSize',14,'FontWeight','normal');
    set(gca,'yscale','log');
    yMin = floor(min(denomin(i,:)));
    yMax = ceil(max(denomin(i,:)));
    set(gca,'xlim',[0 60],'xtick',[0:20:60],'ylim',[1e-2 1e4],'ytick',10.^(-2:2:4));
    set(gca,'color','w','Fontsize',12,'LineWidth',1,'Fontweight','normal');
    set(gca,'box','off','Fontname','Arial','Fontsmoothing','on');
    set(gca,'xgrid','on','ygrid','on','gridcolor','k');
    legend('R_{eff}= 4\mum','R_{eff}= 8\mum','R_{eff}= 13\mum','R_{eff}= 18\mum','location','Northeast');% we need to calculate Reff from Rm using formaula
    legend boxoff
    caption = sprintf('S_{0}=(1,1,0,0)');
    dim = [0.4 .49 .2 .42];%[left bottom width height]
    annotation('textbox',dim,'String',caption,'FitBoxToText','on','FontSize',14,'FontWeight','normal','FontName',...
        'Arial','EdgeColor','w');
    nIDs = 1;
    alphabet = ('a':'z').';
    chars = num2cell(alphabet(1:nIDs));
    chars = chars.';
    charlbl = strcat('(',chars,')'); % {'(a)','(b)','(c)','(d)'}
    text(0.03,0.95,charlbl{1},'Units','normalized','FontSize',14)
    
    %figure(2)
    hold on
    subplot(1,3,2);
    plot(theta,denomin(i,:),'color',col(i),'LineWidth',2);
    xlabel('\fontname{Arial}Scattering angle \theta(\circ)','FontSize',14,'FontWeight','normal');
    %ylabel('\fontname{Arial}Phase function P_{L\mid\mid}(\theta)(sr^-^1)','FontSize',14,'FontWeight','normal');
    set(gca,'yscale','log');
    yMin = floor(min(denomin(i,:)));
    yMax = ceil(max(denomin(i,:)));
    set(gca,'xlim',[60 120],'xtick',[60:20:120],'ylim',[1e-4 1e-1],'ytick',10.^(-4:1:-1));
    set(gca,'color','w','Fontsize',12,'LineWidth',1,'Fontweight','normal');
    set(gca,'box','off','Fontname','Arial','Fontsmoothing','on');
    set(gca,'xgrid','on','ygrid','on','gridcolor','k');
    legend('R_{eff}= 4\mum','R_{eff}= 8\mum','R_{eff}= 13\mum','R_{eff}= 18\mum','location','Northeast');% we need to calculate Reff from Rm using formaula
    legend boxoff
    caption = sprintf('S_{0}=(1,1,0,0)');
    dim = [.2 .49 .2 .42];%[left bottom width height]
    annotation('textbox',dim,'String',caption,'FitBoxToText','on','FontSize',14,'FontWeight','normal','FontName',...
        'Arial','EdgeColor','w');
    nIDs = 2;
    alphabet = ('a':'z').';
    chars = num2cell(alphabet(1:nIDs));
    chars = chars.';
    charlbl = strcat('(',chars,')'); % {'(a)','(b)','(c)','(d)'}
    text(0.03,0.95,charlbl{2},'Units','normalized','FontSize',14)
    
    %figure(3)
    hold on
    subplot(1,3,3);
    plot(theta,denomin(i,:),'color',col(i),'LineWidth',2);
    xlabel('\fontname{Arial}Scattering angle \theta(\circ)','FontSize',14,'FontWeight','normal');
    %ylabel('\fontname{Arial}Phase function P_{L\mid\mid}(\theta)(sr^-^1)','FontSize',14,'FontWeight','normal');
    set(gca,'yscale','log');
    yMin = floor(min(denomin(i,:)));
    yMax = ceil(max(denomin(i,:)));
    set(gca,'xlim',[120 180],'xtick',[120:20:180],'ylim',[1e-4 1e-1],'ytick',10.^(-4:1:-1));
    set(gca,'color','w','Fontsize',12,'LineWidth',1,'Fontweight','normal');
    set(gca,'box','off','Fontname','Arial','Fontsmoothing','on');
    set(gca,'xgrid','on','ygrid','on','gridcolor','k');
    legend('R_{eff}= 4\mum','R_{eff}= 8\mum','R_{eff}= 13\mum','R_{eff}= 18\mum','location','Northeast');% we need to calculate Reff from Rm using formaula
    legend boxoff
    caption = sprintf('S_{0}=(1,1,0,0)');
    dim = [.1 .49 .2 .42];%[left bottom width height]
    annotation('textbox',dim,'String',caption,'FitBoxToText','on','FontSize',14,'FontWeight','normal','FontName',...
        'Arial','EdgeColor','w');
    nIDs = 3;
    alphabet = ('a':'z').';
    chars = num2cell(alphabet(1:nIDs));
    chars = chars.';
    charlbl = strcat('(',chars,')'); % {'(a)','(b)','(c)','(d)'}
    text(0.05,0.95,charlbl{3},'Units','normalized','FontSize',14)
    % Enlarge figure to full screen.
    set(gcf, 'units','normalized','outerposition',[.1 .4 .8 .5]);
end

Answer 1

[Cris LaPierre]

Try using the text command, which lets you specify location.

text(x,y,txt)

For your subplot spacing, try using tilelayout instead. It has optional inputs for TileSpacing and Padding.

t = tiledlayout(1,3,'TileSpacing','Compact','Padding','Compact');