I want to use a low-pass filter to cut off frequencies above the cutoff frequency.

I am trying to cut frequencies above the cutoff frequency using the lowpass function, but it is not working.
I am trying to cut the frequency by applying a lowpass filter to the tunnel current z multiplied by the reference signal sin(wt).
The cutoff frequency is set at 500 Hz, so when I graph the FFT, the peaks at frequencies higher than 500 Hz should disappear, but they do not.

%Parameter Setting
pixel_image = 256; %Input the number of pixels in the image obtained by raster scanning (input 2^n)
dr = 1/(2*sqrt(3)); %Enter dither circle radius [grid].
a_fast_grid = 10; %fast axis scanning range [grid]
a_slow_grid = 10; %Slow axis scanning range [grid]
fm=5000; %Dither circle modulation frequency [Hz]
fs= fm*240 ; %Sampling frequency [Hz]
f_fast = 10.2; %Input scanning frequency [Hz] (1 line scanning count in 1[s])
start_point_x = 0; %Input x-coordinate of scanning start point (input 1 if you want to move by 1[grid])
start_point_y = 0; %Input y-coordinate of scanning start point (input 1 if you want to move by 1[grid])

%Parameter setting for fast-axis triangular wave
amplitude_fast = a_fast_grid/2; %fast axis amplitude

%Parameter setting for slow-axis triangular wave
amplitude_slow = a_slow_grid/2; %slow axis amplitude
f_slow = (f_fast)/(2*pixel_image); %Slow axis triangular wave frequency

% Generation of time vectors
total_time=256/f_fast; %Total Scan Time
t = linspace(0, total_time, fs * total_time);

x_raster = start_point_x + amplitude_fast*(2/pi)*acos(cos(2*pi*f_fast*t));
y_raster = start_point_y + amplitude_slow*(2/pi)*acos(cos(2*pi*f_slow*t));

x_dither = dr*cos(2*pi*fm*t);
y_dither = dr*sin(2*pi*fm*t);

x = x_raster + x_dither;
y = y_raster + y_dither;

z1 = cos(2*pi*((x-y)/(sqrt(3))));
z2 = cos(2*pi*(2*y/(sqrt(3))));
z3 = cos(2*pi*((x+y)/(sqrt(3))));
z = (z1 + z2 + z3);

%Reference Signal Creation
w = 2*pi*fm ;
phi = 0 ;
reference_signal = sin(w*t+phi) ;

%Mixing signal creation
mixising_signal = z.* reference_signal ;

%Applying a low-pass filter
f_cutoff = 500 ; %Cutoff frequency [Hz] setting
lowpass_signal = lowpass(mixising_signal,f_cutoff,fs) ;

%The result of FFT.——————————————-
n = length(t); % Number of samples
f = (0:n-1)*(fs/n); % Frequency axis [Hz]
half_n = floor(n/2); % Half of data

fft_z = abs(fft(z) / n);
fft_mixising_signal = abs(fft(mixising_signal) / n);
fft_lowpass_signal = abs(fft(lowpass_signal) / n);

figure;
tiledlayout(3,1)
nexttile
plot(f(1:half_n), fft_z(1:half_n));
title(‘FFT of tunnel current’); xlabel(‘Frequency [Hz]’); ylabel(‘Amplitude’);
xlim([0 20000]),ylim([0 0.3]);

nexttile
plot(f(1:half_n), fft_mixising_signal(1:half_n));
title(‘FFT of mixing signal’); xlabel(‘Frequency [Hz]’); ylabel(‘Amplitude’);
xlim([0 20000]),ylim([0 0.3]);

nexttile
plot(f(1:half_n), fft_lowpass_signal(1:half_n));
title(‘FFT after applying low-pass’); xlabel(‘Frequency [Hz]’); ylabel(‘Amplitude’);
xlim([0 20000]),ylim([0 0.3]);I am trying to cut frequencies above the cutoff frequency using the lowpass function, but it is not working.
I am trying to cut the frequency by applying a lowpass filter to the tunnel current z multiplied by the reference signal sin(wt).
The cutoff frequency is set at 500 Hz, so when I graph the FFT, the peaks at frequencies higher than 500 Hz should disappear, but they do not.